(certified from an accredited laboratory. 5 -year stability and easy handling) features: convenient 250 ml and 500 ml plastic bottle with built-in calibration compartmenteconomical, since only about 20 ml of buffer is used per calibrationactual value is determined by a dkd laboratory, accredited for ph measurementfirst class certificate with traceability to international standardsexpiration date on the bottleimmune to micro-organisms phbuffers available as: solution, tablets, powders & concentrates